1. Field of the Invention
This invention relates generally to construction machines, and more particularly, but not by way of limitation, to improved braking systems for hydraulically powered road rollers.
2. Description of the Prior Art
The invention relates to a braking system for construction machines, in particular for road rollers having hydraulic propelling drive, wherein brakes, which are biased in brake-applying direction by mechanical means, are held in released state by brake releasing pressure in a brake releasing circuit by means of hydraulic actuating members, the brake releasing pressure being generated by an operational releasing pressure pump which together with a hydraulic pump for the propelling drive is coupled to a prime mover, wherein the brake releasing circuit is arranged to be connected to the reservoir through an electrically controlled valve which is held in its position closing the brake releasing circuit by electric auxiliary power, wherein, in addition, a shunt conduit is provided between the inlet and outlet ports of the propelling drive hydraulic pump and contains a first shut-off valve, wherein furthermore the connecting conduit from the operational releasing pressure pump to the hydraulic actuating members is arranged to be shut off by a second shut-off valve arranged downstream of the electrically controlled valve, and wherein an auxiliary releasing pressure pump arranged to be actuated by muscular power is connected to the brake releasing circuit downstream of the second shut-off valve.
Construction machines, in particular road rollers, having hydraulic propelling drive comprises a hydraulic pump, which is coupled to the prime mover, as well as hydraulic motors on the driven wheels or roller drums. The discharge volume of the hydraulic pump which is usually an axial piston pump, is variable, in order to be able to regulate the travelling speed. When the hydraulic pump is set to zero stroke, it will, at the same time, act as a brake. Namely if the roller drum or wheels would tend to continue to roll with zero stroke of the axial piston pump, the hydraulic motors coupled thereto would tend to act as pumps and to supply oil to the hydraulic pump, now acting as a hydraulic motor. As, however, the intake volume of this "motor" is zero, the roller drums or wheels are hydraulically locked through the hydraulic motors. In normal operation such a construction machine having hydraulic propelling drive is braked by means of the propelling drive itself by varying the hydraulic pump.
For safety reasons, however, an additional braking system is provided, which permits "emergency braking", if, for example, the hydraulic system of the propelling drive becomes leaky on a slope and is no longer able to take charge of this braking function. Apart from this, this additional braking system can serve to initiate automatic braking upon occurrence of other conditions, if, for example, the operator leaves the construction machine, while it is in motion. The additional braking system comprises brakes which are biased in brake-applying direction by mechanical means and which are held in their released states by brake releasing pressure in a brake releasing circuit by means of hydraulic actuating members. Actuation of these brakes is effected purely mechanically without hydraulic auxiliary power being required. Actuation of the brakes is effected also and just in that case, when the hydraulic auxiliary power fails. The actuation of this additional braking system is effected by means of a valve through which the brake releasing circuit can be connected to the reservoir. As this valve is to be controlled depending on various functions, it is constructed as an electrically controlled valve which is retained by electric auxiliary power in its position closing the brake releasing circuit. The brake releasing pressure is generated by an operational releasing pressure pump, which together with the hydraulic pump for the propelling drive is coupled to the prime mover.
With such an arrangement problems would arise if the hydraulic and/or electric auxiliary power of the construction machine had failed and the construction machine were to be towed away. As the hydraulic pump for the propelling drive is standing still with the prime mover not running and therefore is not able to accommodate any oil, and, on the other hand, the drive motors would act as pumps, when the construction machine is towed, there would be hydraulic locking of the roller drums or wheels by the hydraulic system of the propelling drive, similar to the setting of the hydraulic pump to zero stroke. In addition the brake releasing pressure would cease and, with failure of the electric auxiliary power, the electrically controlled valve would connect the brake releasing circuit to the reservoir. Thereby the brakes of the additional braking system would be applied.
This is prevented by particular measures to permit towing.
Firstly a shunt conduit between inlet and outlet ports of the propelling drive hydraulic pump is provided. A first shut-off valve, which is normally closed, is located in this shunt conduit. By opening this shut-off valve the hydraulic pump of the propelling drive can be shunted, whereby the locking of the hydraulic motors is eliminated.
Furthermore a second shut-off valve is provided, by which the connecting conduit from the operative releasing pressure pump to the hydraulic actuating members downstream of the electrically controlled valve can be shut off. Thereby communication between the brake releasing circuit, on one hand, and both the operative releasing pressure pump and the electrically controlled valve, is interrupted which is connected to reservoir even with failure of the current supply. An auxiliary releasing pressure pump arranged to be actuated by muscular power is connected to the brake releasing circuit downstream of the second shut-off valve. This auxiliary releasing pressure pump permits building up a pressure in the brake releasing circuit independently of any auxiliary power, and releasing the brakes of the additional braking system.
Then the construction machine can be towed.
With this prior art arrangement dangerous situations due to faulty manipulation can occur. Namely if after towing and restarting of the construction machine the first shut-off valve is closed again but the operator forgets to open the second shut-off valve again, the brakes of the additional braking system will remain released. The construction machine can be operated normally with the propelling drive and can be braked, if necessary. However it is not possible to actuate the additional braking system by changing the electrically controlled valve over, as the second shut-off valve is closed between this electrically controlled valve and the brake releasing circuit proper. This deficiency will remain unnoticed at first. In a dangerous situation, however, it will be found impossible to apply the brakes of the additional braking system. The second shut-off valve has, however, to be located downstream of the electrically controlled valve, as otherwise it would not be possible to shut the brake releasing circuit off towards the reservoir in the case of a failure of the electric auxiliary power.